The stalemate over the debt conjures up a lot more dirty words about Congress, but it is also the norm, the latest fault line in a decades-long trend that has virtually wiped out the political center.

Democrats are more liberal and Republicans are more conservative, each side clinging to ideological lines in the marble floors of the Capitol. Don't you dare touch Medicare or Social Security, Democrats say. New taxes? We would rather shut down the government, insist Republicans, who are locked in by antitax pledges.

Bipartisanship exists in spurts and there's hope still that a deal can be reached to avert a government default on its obligations. (There was little progress Wednesday as the GOP-controlled House revised its plan and the Democratic-controlled Senate did the same, both hoping to increase chance of passage.)

No matter what happens, the polarization has come into full view.

"As conservative as I was, I could get along with a liberal. You've got to listen to know what they're thinking," said former U.S. Rep. Clay Shaw, who served his South Florida district for 26 years before losing in the Democratic wave of 2006 that made Nancy Pelosi speaker of the House. "Now nobody listens to anybody. We're losing the middle, and that's what you need to make deals."

The erosion began in the 1980s and has continued as conservative Democratic voters in the South have shifted to the GOP and liberal Republican lawmakers in the northeast retired, or were defeated or forced from their party for not being conservative enough.

The 2006 and 2008 elections brought waves of liberal Democrats who used their new majority and control of the White House to push a broad agenda including the economic stimulus and health care bill that had almost no support from Republicans.

The GOP rallied in opposition, rejecting compromise at virtually every turn, and won control of the House in 2010. The election ushered in the tea party era and further cut the ranks of conservative Blue Dog Democrats, once a formidable caucus, now an afterthought.

The freshman class of 87 Republicans viewed the election as a mandate to stick to a hard right agenda. Rep. Allen West of South Florida summed it up this week in a conference call with conservatives: "This is a seminal moment where we cannot have any retreat, no surrender."

The tea party has pushed Republicans further right, complicating House Speaker John Boehner's effort at compromise. He was struggling to gain GOP support for the debt-cutting plan that Senate Democrats uniformly vowed to oppose.

Frustrations boiled over Wednesday with Sen. John McCain, R-Ariz., who accused the new House members of making a fiscally and politically dangerous stand.

"It's time we listened to the markets," he said. "It's time we listened to our constituents. But most of all, it's time we listened to the American people and sit down and seriously negotiate something."

Taxes are the primary sticking point for Republicans. Democrats sought some revenue increases to go along with spending cuts — Senate Majority Leader Harry Reid's newest plan does not include any new taxes — but were met with vociferous opposition.

That stance is driven by a belief that taxes would harm the economy, but also the antitax pledge most Republicans have signed. To go back on the pledge would be to invite not only the wrath of Grover Norquist, president of the Americans for Tax Reform, but the label of RINO — Republican In Name Only.

Democrats send up similar howls when changes to Medicare or Social Security are discussed. Obama has angered his base by offering up changes to entitlements, undercutting an election-year attack strategy for his party.

"It's easy to reach agreement if the differences are purely about policy," said Sarah Binder, a senior fellow at the Brookings Institution who has studied the partisan gulf.

"If one party wants $20 billion in savings and another wants $40 billion, it's usually not too difficult to meet in the middle. But if the reason you disagree is strategic, how do you compromise? No one wants a half a loaf."

A big factor in all this: Congressional districts are less competitive.

Population shifts and redistricting has made for distinctly partisan territories, leaving little incentive for elected officials to stray from the party line. Those in the safest districts tend to climb leadership ranks because they are free to raise money for other candidates, collecting IOUs along the way, and instill their views in a top-down system.

"You try to be more inclusive, and in the end it doesn't matter because you're forced by the institution to be on one side or the other," said Dan Maffei, a moderate Democratic representative from upstate New York who was defeated by a Republican after one term in 2010.

The real threat for many House members is a party primary. "When you're worried more about your left or right than the opposite party, you tend to take a hard line vote," said Republican pollster David Winston.

The fighting over the debt in Washington has angered voters nationwide. They want compromise. Many want a balanced approach of revenue cuts and tax increases.

But voters contribute to the divide. Americans are "self-sorting" into like-minded communities, said David Wasserman, who studies House elections for the Cook Political Report. An analysis of precincts in various states showed that over the past 20 years there has been a 15 percent decline of the number ofvoters living in moderateswing areas.

People have moved from cities and made suburbs more racially diverse. But the moves also have created either heavily minority (Democratic) areas or heavily white (Republican) areas. "We can blame polarization all we want on politicians in Washington," Wasserman said, "but the true cause is right in the mirror."